Biomass production and nitrogen accumulation by hairy vetch/cereal rye mixtures: A meta-analysis

Authors: THAPA, RESHAM - University Of Maryland; POFFENBARGER, HANNA - Iowa State University; TULLY, KATHERINE - University Of Maryland; ACKROYD, VICTORIA - University Of Maryland; Kramer, Matthew; Mirsky, Steven

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2018
Publication Date: 6/21/2018
Citation: Thapa, R., Poffenbarger, H., Tully, K., Ackroyd, V.J., Kramer, M.H., Mirsky, S.B. 2018. Biomass production and nitrogen accumulation by hairy vetch/cereal rye mixtures: A meta-analysis. Agronomy Journal. 110(4):1197-1208. https://doi.org/10.2134/agronj2017.09.0544.
DOI: https://doi.org/10.2134/agronj2017.09.0544

Interpretive Summary: Cover crops are planted for the benefits they provide to a cropping system, rather than for harvest. As a result, cover crops are typically grown when the ground would otherwise be bare (i.e., late fall – spring). Benefits provided by cover crops include weed suppression (because the cover crops outcompete weeds for space, nutrients, and light), and nitrogen provided by legume cover crops that fix atmospheric nitrogen. Cover crops provide such benefits in varying degrees. For example, grasses like cereal rye excel at weed suppression because they are quick to grow in the fall and produce large amounts of biomass. However, grasses do not provide nitrogen beyond what is already in the soil for them to accumulate. Legumes like hairy vetch potentially provide large amounts of nitrogen fixed from the atmosphere, but are slow to establish in the fall, and thus are not as effective as grasses at weed suppression. Farmers often grow a mixture of cover crop species to take advantage of the benefits provided by multiple species. Cereal rye combined with hairy vetch (cereal rye/hairy vetch) is the most commonly grown two-species mixture. The proportion of each species in a mixture significantly impacts the amount of biomass and nitrogen content produced by that mixture. Much research has been done to quantify the impact of cereal rye/hairy vetch mixtures vs. pure stands of each cover crop on weed suppression and nitrogen accumulation, to determine if it is worth the extra seed cost to grow a mixture. We conducted a meta-analysis of 20 such field studies. Meta-analysis provides insights that can’t be gathered from individual studies – by aggregating data from multiple studies, we benefit from more powerful statistical tools that allow us to draw conclusions in spite of varying conditions found within a given study (e.g., weather differences year to year in a study). Cereal rye/hairy vetch mixtures produced more biomass than pure stands of either cover crop, indicating the potential value of growing a mixture (which will suppress weeds and accumulate more nitrogen because of the larger biomass produced compared to the pure stands). Mixtures produced 1.6 times more nitrogen than pure stands of cereal rye, indicating the value of growing hairy vetch with cereal rye. We did find these results to be dependent on soil type, previous crop, seeding proportion of cereal rye and hairy vetch in mixture, and weather. For example, we found that under conditions in which nitrogen was limited (such as on less-productive, sandy soils), cereal rye/hairy vetch mixtures were more likely to produce more biomass and accumulate more nitrogen than pure stands of either cover crop. This work demonstrates the value of cereal rye/hairy vetch mixtures, outlines the environmental circumstances under which mixtures are most likely to produce enough biomass and accumulate enough nitrogen to meet farmer needs (e.g., weed suppression and improved fertility), and can help serve as the basis for recommendations to improve cropping system sustainability through the use of cover crops. This information will be useful to scientists devising strategies to integrate cover crops into crop production systems.

Technical Abstract: Cover crop mixtures are increasingly employed to provision multiple agroecosystem services. However, there is inconsistent evidence in the literature on biomass production and nitrogen (N) accumulation by individual cover crop species as compared to when grown in mixtures. We conducted a meta-analysis using results from 20 studies to examine biomass and N content of hairy vetch (Vicia villosa Roth)/cereal rye (Secale cereale L.) mixtures compared to hairy vetch, cereal rye, and the max monocultures. ‘Max monocultures’ refers to the monoculture with greatest biomass (cereal rye in 71% of the cases) or highest N content (hairy vetch in 88% of the cases). Overall, hairy vetch/cereal rye mixtures produced 60, 25, and 7% more biomass when compared to hairy vetch, cereal rye, and the max monocultures, respectively. The N content of hairy vetch/cereal rye mixtures was 1.6 times more than that in cereal rye monocultures. Results further indicated that the relative productivity of hairy vetch/cereal rye mixtures depends on soil type, previous crop, seeding proportion, and growing degree days (GDD). Compared to monocultures, a more consistent positive response of mixture biomass and N content was found under N-limiting conditions (on coarse-textured soils and following corn than soybean harvest). With increasing GDD, the biomass and N content of mixtures increased relative to cereal rye monocultures but decreased relative to hairy vetch monocultures, suggesting better performance of hairy vetch at higher GDD. When the proportion of hairy vetch exceeded 48% of the mixture, the mixtures accumulated equivalent or relatively more N than the legume monoculture and the max monocultures. The consistently higher biomass productivity of hairy vetch/cereal rye mixtures, compared to the monocultures, further underscores the value of combining grass and legume cover crop species when maximum biomass is the objective. Furthermore, cereal rye/hairy vetch cover crop mixtures can produce comparable total N content to monoculture legumes.